I installed another laptop with Win XP Pro SP3. I got the plugin up and running in 5 minutes. Encouraging, but frustrating at the same time.
I read the thread you mentioned in post 2 and followed the same steps as Helmsmatt at the very beginning of my investigations. Basically, it is about defining rules or exceptions in the firewall. On top of that, there are many differences between the home and pro editions of win7 in regard to networking.
I would like to identify what is blocking the communication but I miss background information on the plugin itself. Is it using only UDP protocol ?
I think it could be valuable for other users to get the right config on win7....

Hi!
I had the same problem. After the countdown the radar were stuck in standby mode.
What I had to do to make it work was to right click the mouse and choose Garminradar control menu item. When the menu pops up the first thing you have to do is to choose master mode(or slave mode if you have a garmin plotter). Then you start the radar with the hardware switch you installed, and the icon in the menu bar will start downcounting. At this point you can hear the radar working. When its done downcounting it goes into standby mode and you cant hear the radar anymore. Dobleclick on the radar icon and it starts working ! :-)